A great effort has been made to enhance power extracted from fiber lasers. Recently, state of the art individual single mode (SM) fiber laser units have proven to be able to deliver up to 3 kW of power. A possible way to deliver higher power is by combining several fiber lasers. Generally, combining several fiber lasers results in outputs with high efficiency, but degraded laser beam quality; vice versa, in order to maintain a high beam quality of a combined beam, only a small amount of the power can be collected and thereby the combining efficiency is degraded.
Commonly available fused fiber combiners utilize multimode entry fibers and a deep multimode output fiber; thus, the major restriction is preventing light from radiating to the surrounding media. The combined structure is obtained by bundling, heating and pulling the individual source fibers together. An optional addition is to splice an output fiber with a mode capacity that can efficiently deliver the bundle output beam power while maintaining its beam quality. This addition may have some engineering benefits such as preferred thermal management and good structural behavior. The fused section is cleaved at a point where its diameter is equal to the inner core of the exit fiber. A connecting splice is done between the reduced bundle and the exit fiber's inner core. The propagating beam is bounded to the core only.
Various methods for combining fiber lasers have been suggested in the past. The principal methods and their limitations are described as follows:
1. Coherent fiber beam combining—in this technique, the temporal phases of the laser sources are matched. The method needs either a self-locking mechanism or a precise control system that corrects the phase mismatch of each individual laser. Until now, either small power was extracted, usually utilizing fiber laser interferometer setups, or medium to high power, in the order of several hundreds Watts with a complicated setup that is hard to implement in an industrial/field environment. Another approach is combining many diode lasers through fibers that are held in front of free space, partially reflecting mirrors, so a mutually phase locking is created between the sources by partial reflection. It has been reported that only a few of the sources are actually locked and there is extreme sensitivity to ambient conditions.
2. Incoherent fiber spectral beam combining: This method uses a highly dispersive element, usually a blazed Bragg grating, or photo-refractive Bragg grating. The individual lasers are designed to be wavelength-shifted from one another. The laser outputs are launched upon the dispersive element and are either reflected or refracted at different angles related to their wavelength. The method suffers from the needs of free-space optics setup, precise angular and translational positioning tolerances and very narrow line width of the sources wavelengths; otherwise the combined beams are not spatially overlapped, and the transfer efficiency of the refractive element decreases. Up to now the state of the art of combined power is approximately 500 W, while maintaining a good beam quality.
3. Direct fiber combining using a multimode fiber bundle and a deep multimode collection (exit) fiber: This technique combines the power of several fiber coupled diode lasers, for example by introducing a “fiber junction” where the beams are brought into close proximity by bundling and tapering the fibers using a hot flame and pulling machine. At a certain stage, the fused bundle is cleaved and attached to the final exit fiber. The exit fiber can contain the whole mixed beams since it is of a multimode type. The common application of this method is combining high power fiber-coupled laser diodes. The combined power levels have reached to the level of near 1 Kw with good power transfer efficiency. Although high power can be combined, the resulting beam quality is relatively low because of the deep multimode nature of the exit (collecting) fiber.
US Patent Application 20100189138 describes a high power fiber laser system that has a combiner including single mode (SM) fibers fused together that define an output end of the fiber combiner. The fused SM fibers radiate respective fiber outputs, which collectively define a multimode (MM) combiner output. The SM fibers each are configured with such optimally small numerical apertures (NA) that the MM combiner output is characterized by a minimally possible beam quality factor (M2) for the plurality of SM fibers. To reduce the possibility of burning the components of the fiber laser system with a multi-kilowatt combiner output, a coreless termination block is fused to the output end of the fiber combiner and configured so as to provide expansion of the combiner output without modifying the minimally possible M2 factor.
US Patent Application 20100189138 limits the combined lasers, which are coupled into each of the input ports, to be purely SM, due to the SM input fibers which are used. However, most high power fiber lasers are characterized by a non-SM output; typically the laser output M2 is ˜1.4, which cannot be coupled efficiently into a purely SM fiber (of which M2 is <1.1). Accordingly, when coupling non-SM lasers to each of the SM inputs, a low coupling efficiency is achieved, which renders the device of US Patent Application 20100189138 to be sub-optimal in terms of efficiency and/or brightness.